When did Britain learn that Japan was building Yamato?

When did the British first learn that Yamato was being planned and built?

They heard of the plans around 1935-36 and of construction in 1937, while expecting roughly 17.5-inch guns.

British networks in Japan reported planning around 1935-36 and construction in 1937. British estimates centred on a 17.5-inch gun, even though Japan had investigated 18-inch weapons since the First World War. This intelligence made the continued British commitment to a 14-inch battleship especially difficult to defend. The King George V turret ring could still have accommodated triple 15-inch or 16-inch mounts in place of quadruple 14-inch turrets.